In the high speed dispensing, or pumping, of precise amounts of liquid for deposition on a substrate, it is common to use an auger, or screw, dispenser. Use of an auger dispenser helps achieve a high degree of repeatability and control of the liquid being dispensed. These features are important because of the high speed nature of this deposition process. For instance, in depositing "drops" of solder paste on a substrate, for some applications it is necessary to dispense at a rate of 25,000 dots per hour, or just less than seven dots per second. Under these conditions a high degree of liquid control is necessary in order to achieve sufficient repeatability and accuracy in deposition.
In an auger dispenser, an auger is held within a pump body. The auger extends along an axial flowpath for the liquid to be dispensed, with an outlet end of the auger located adjacent an exit port of the flowpath and an opposite, drive end of the auger held by and rotatably driven by a coupling, which operatively connects to a drive motor via a drive shaft motor. The pump body has an inlet in fluid communication with the flowpath, and a liquid supply line supplies dispensing liquid to the flowpath. Initially, liquid is supplied to the flowpath under pressure, to fill the flowpath along the length of the auger, and then the pressure is reduced to a lower operating level. Thereafter, incremental rotations of the drive motor cause rotation of the auger, thereby producing drops of liquid, such as solder paste, to flow from the exit port for deposition on a substrate.
In some instances, angular rotation of the motor drive shaft of about 1/16th of a full rotation results in a drop of liquid exiting from the exit port. With movement of the substrate relative to the exit port of the dispenser, which is usually held in a fixed position relative to the travel path of the substrate, discrete successive partial rotations of the motor shaft, and the auger coupled thereto, results in repeated deposition of drops of liquid on the substrate.
In one prior version of an auger dispenser of this type, the structural components of the pump body, including the components which hold the auger, are held together by bolts and set screws, to promote rigidity and stability during use. While this structure itself has proved quite suitable for dispensing applications, the disassembly of the structure for cleaning purposes can be time consuming and tedious. This results in greater downtime for the dispenser and lower overall throughput for the dispenser.
In a different, but prior version of an auger dispenser used for similar applications, the auger resides within a cartridge which removably connects to the remainder of the pump body, or dispenser structure, via radial movement of a threaded, manually operable set screw which threads radially into position from the pump body to engage an axially oriented slot in the exterior of the cartridge. The axial orientation of the slot enables the set screw to move axially, so that the connected cartridge and the auger held therein are axially movable relative to the remainder of the dispenser structure, including the coupling.
Even with this removable cartridge structure, connection and disconnection of the cartridge requires manual rotation of the set screw to engage or disengage the slot. Moreover, the axial slot formed in the cartridge necessarily permits axial movement of the cartridge and the auger relative to the other pump components, a structural arrangement which is suitable for "floating head" dispensing, wherein the pump body moves to and from the substrate and actually contacts the substrate.
Because there are some applications for auger dispensers wherein the dispensing head does not "float," i.e. wherein the cartridge, the auger and the rest of the pump components may remain axially fixed, there are thus applications where this relative movement between the coupling and the auger is unnecessary. Even for floating head operation, axial movement of the auger relative to other pump components may not be desirable, for the reasons described below.
In "floating head" deposition of liquid on a substrate, the pump body moves toward the substrate until a standoff, carried by the pump body, contacts the substrate. Liquid dispensing is correlated to contact between the standoff and the substrate. This occurs repeatedly during liquid deposition. The pump body is moved toward the substrate, a liquid drop is deposited on the substrate, and then the pump body moves away from the substrate.
The pump body carries the standoff, and the other pump components carried by the body also move to and from the substrate, including the auger. However, when this structure contacts the substrate, as is typical during "floating head" operation, some structure along the dispenser axis must absorb this impact. This usually means that the coupling structure, which operatively connects the drive shaft to the auger, must accommodate relative movement along the axis of dispensing, and also must repeatedly absorb the impact with the substrate. Typically, the coupling is housed with the pump body and includes an axially elongated keyway sized to coact, or receive, an end of the auger, thereby to transmit torque thereto relative to the dispensing axis and to accommodate axial movement of the auger relative to the coupling.
In some instances, the coupling may actually be rotating and transmitting torque to the auger while the auger is moving axially, relative to the coupling, and during contact of the pump body with the substrate. This presents several problems. The surface to surface contact between the auger and the coupling generates wear, and can even result in the grinding away of small particles. These particles can get into the dispensing liquid. Also, because of the need for the coupling and the auger to accommodate relative axial movement therebetween, during manufacture both must be machined with longer clearances. These greater clearances result in less-effective torque transmission to the auger. Moreover, if relative axial movement occurs during torque transmission, the auger may bind somewhat within the coupling, which causes excessive surface to surface wear and/or may inhibit axial movement. This can also result in excessive impact force to the substrate. Because of the need for high speed, high precision and a high degree of repeatability in some liquid dispensing applications, as for instance the deposition of solder paste on a substrate, these disadvantages are not acceptable.
In typical floating head dispensers, the dispensing head (which in this case includes the pump body) is movable relative to the rest of the pump due to the use of a slide bearing, with the slide bearing including a bushing surrounding, or at least engaging, the pump body fairly close to the dispensing axis. With these structures, to minimize excessive wear it is necessary to increase the clearance for the bearing surfaces. However, the increased clearances inevitably result in a less precise movement of the pump body toward and away from the substrate. On the other hand, reducing the clearance would produce an increase in wear due to surface to surface contact between the pump body and the bearing.
In addition to these concerns relative to "wear" and clearance, with these dispensers it is also important to keep in mind that overall efficiency in operation, i.e. decreased downtime and increased throughput, can be achieved only if the components can be assembled and disassembled relatively quickly and easily.
Also, certain liquids dispensed by pumps of this type, such as solder paste, may require special handling considerations. For instance, solder paste is sensitive to changes in fluid directional flow. Such directional changes may cause the solder to cold weld in place, thereby clogging the dispenser.
It is an object of this invention to reduce the magnitude of impact forces applied to a substrate and the wear which results from axial movement of a pump body of a floating head auger pump, without sacrificing precision or repeatability in liquid dispensing.
It is another object of the invention to improve the torque transmission between a drive shaft and an auger for a floating head auger dispenser, without increasing wear between components.
It is still another object of the invention to further improve the precision and repeatability of liquid dispensing for an auger dispenser, without compromising simplicity in design and without sacrificing convenience relative to assembly and disassembly of the dispenser components, while also minimizing the occurrence of cold welding of the liquid being dispensed.